Adjustable tuning of a dielectrically loaded loop antenna

ABSTRACT

A microwave antenna assembly is disclosed. The antenna assembly includes an elongated member defining a longitudinal axis and having proximal and distal ends. The antenna assembly also includes an outer conductor and an inner conductor each disposed within the elongated member and extending along the longitudinal axis. A portion of the inner conductor is deployable relative to the outer conductor such that the antenna assembly may transition from a first configuration to a second configuration. The antenna assembly also includes an expandable sheath at least partially disposed about a distal portion of the inner conductor and defining at one or more lumens configured to couple to a supply of dielectric material used to regulate the expansion of the expandable sheath.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patent application Ser. No. 14/281,344 filed on May 19, 2014, which is a continuation application of U.S. patent application Ser. No. 12/826,897 filed on Jun. 30, 2010, now U.S. Pat. No. 8,740,893, the entire contents of each of which are incorporated herein by reference.

BACKGROUND

1. Technical Field

The present disclosure relates generally to microwave antennas. More particularly, the present disclosure is directed to flexible loop antenna having a variable dielectric loading.

2. Background of Related Art

Treatment of certain diseases requires destruction of malignant tissue growths (e.g., tumors). It is known that tumor cells denature at elevated temperatures that are slightly lower than temperatures injurious to surrounding healthy cells. Therefore, known treatment methods, such as hyperthermia therapy, heat tumor cells to temperatures above 41° C., while maintaining adjacent healthy cells at lower temperatures to avoid irreversible cell damage. Such methods involve applying electromagnetic radiation to heat tissue and include ablation and coagulation of tissue. In particular, microwave energy is used to coagulate and/or ablate tissue to denature or kill the cancerous cells.

Microwave energy is applied via microwave ablation antennas that penetrate tissue to reach tumors. There are several types of microwave antennas, such as monopole and dipole, in which microwave energy radiates perpendicularly from the axis of the conductor. A monopole antenna includes a single, elongated microwave conductor whereas a dipole antenna includes two conductors. In a dipole antenna, the conductors may be in a coaxial configuration including an inner conductor and an outer conductor separated by a dielectric portion. More specifically, dipole microwave antennas may have a long, thin inner conductor that extends along a longitudinal axis of the antenna and is surrounded by an outer conductor. In certain variations, a portion or portions of the outer conductor may be selectively removed to provide more effective outward radiation of energy. This type of microwave antenna construction is typically referred to as a “leaky waveguide” or “leaky coaxial” antenna.

SUMMARY

According to one embodiment of the present disclosure, a microwave antenna assembly is disclosed. The antenna assembly includes an elongated member defining a longitudinal axis and having proximal and distal ends. The antenna assembly also includes an outer conductor and an inner conductor each disposed within the elongated member and extending along the longitudinal axis. A portion of the inner conductor is deployable relative to the outer conductor such that the antenna assembly may transition from a first configuration to a second configuration. The antenna assembly also includes an expandable sheath at least partially disposed about a distal portion of the inner conductor and defining at least one lumen configured to couple to a supply of dielectric material used to selectively expand the sheath.

According to another embodiment of the present disclosure, a microwave ablation system is disclosed. The system includes an antenna assembly having an elongated member defining a longitudinal axis and having proximal and distal ends. The antenna assembly also includes an outer conductor and an inner conductor each disposed within the elongated member and extending along the longitudinal axis. A portion of the inner conductor is deployable relative to the outer conductor such that the antenna assembly may transition from a first configuration to a second configuration. The antenna assembly also includes an expandable sheath at least partially disposed about a distal portion of the inner conductor and defining at least one lumen configured to receive a dielectric material. The system includes a fill source coupled to the lumen and configured to regulate the amount of the dielectric material within the lumen to control the expansion of the expandable sheath.

A method for performing microwave ablation is also contemplated by the present disclosure. The method includes the initial step of inserting an antenna assembly into a tissue volume. The antenna assembly includes an outer conductor, an inner conductor and an expandable sheath at least partially disposed about a distal portion of the inner conductor and defining at least one lumen. The method also includes the steps of deploying at least a portion of the inner conductor from the outer conductor such that the antenna assembly may transition from a first configuration to a second configuration and regulating an amount of a dielectric material supplied to the at least one lumen to control expansion of the expandable sheath. The method further includes the steps of energizing the antenna assembly to ablate the tissue volume and withdrawing the dielectric material from the at least one lumen to deflate the expandable sheath.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of the present disclosure will become more apparent in light of the following detailed description when taken in conjunction with the accompanying drawings in which:

FIG. 1 is a perspective view of a microwave ablation system according to an embodiment of the present disclosure;

FIGS. 2A-2B are cross-sectional views of a feedline according to an embodiment of the present disclosure;

FIG. 3 is a partial, cross-sectional view of a microwave antenna assembly according to an embodiment of the present disclosure;

FIG. 4 is a partial, cross-sectional view of the microwave antenna assembly of FIG. 2 in a partially deployed configuration according to an embodiment of the present disclosure;

FIG. 5 is a top view of the microwave antenna assembly of FIG. 2 in a fully deployed configuration according to an embodiment of the present disclosure;

FIG. 6 is a cross-sectional view of the microwave antenna assembly of FIG. 5 according to an embodiment of the present disclosure;

FIG. 7 is a cross-sectional view of another embodiment of a microwave antenna assembly according to an embodiment of the present disclosure;

FIG. 8 is a cross-sectional view of another embodiment of a microwave antenna assembly according to an embodiment of the present disclosure;

FIG. 9 is a cross-sectional view of another embodiment of a microwave antenna assembly according to an embodiment of the present disclosure;

FIG. 10 is a top view of a microwave antenna assembly in a fully deployed configuration according to an embodiment of the present disclosure;

FIG. 11 is a cross-sectional view of the microwave antenna assembly of FIG. 10 in a deflated configuration according to an embodiment of the present disclosure;

FIG. 12 is a cross-sectional view of the microwave antenna assembly of FIG. 10 in an inflated configuration according to an embodiment of the present disclosure;

FIG. 13 is a cross-sectional view of another embodiment of a microwave antenna assembly in a deflated configuration according to an embodiment of the present disclosure;

FIG. 14 is a cross-sectional view of the microwave antenna assembly of FIG. 13 in an inflated configuration according to an embodiment of the present disclosure; and

FIG. 15 is a flow chart of a method according to an embodiment of the present disclosure.

DETAILED DESCRIPTION

Particular embodiments of the present disclosure are described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail to avoid obscuring the present disclosure in unnecessary detail. In the drawings and in the description that follows, the term “proximal,” as is traditional, will refer to the end of the apparatus that is closest to the clinician, while the term “distal” will refer to the end that is furthest from the clinician.

Referring to FIG. 1, a microwave tissue treatment system 10 in accordance with an embodiment of the present disclosure is shown. System 10 includes a microwave antenna assembly 100 connected to a power source or supply 20, e.g., a microwave or RF generator or any suitable power generating device suitable for energizing the antenna assembly 100, through a feedline 30. The power supply 20 is configured to provide microwave energy at an operational frequency from about 300 MHz to about 10,000 MHz.

The system 10 also includes a fill source 40, e.g., an electric motor pump, a peristaltic pump or the like, as a mechanism for circulating a dielectric material “M,” such as gas (e.g., nitrogen, air, etc.) or liquid (e.g., saline, water, etc.) through the antenna assembly 100, as described below. Antenna assembly 100 may further include a pusher or deployment assembly 50 that includes a deployment knob 52 operatively engaged with or coupled to the antenna assembly 100, as described in further detail below.

Referring now to FIGS. 1-2B, as indicated above, antenna assembly 100 is electrically connected to generator or power supply 20 by feedline 30. Feedline 30 may be any suitable conductive pathway capable of transferring an electrical current to antenna assembly 100. In one embodiment, as seen in FIGS. 2A-2B, feedline 30 may be a coaxial cable composed of an inner conductor 102, an outer conductor 104, and an inner insulator 106 interposed between inner and outer conductors 102, 104 to electrically separate and/or isolate inner and outer conductors 102,104 from one another. Inner and outer conductors 102, 104 may each be made of a suitable conductive material that may be semi-rigid or flexible, while inner insulator 106 may include any number of suitable non-conductive materials such as ceramic and polytetrafluoroethylene (PTFE). Inner and outer conductors 102, 104 of feedline 30 may incorporate any suitable conductive material or metal, including, but not limited to, silver, copper and gold. In certain embodiments, inner and outer conductors 102, 104 of feedline 30 may include a conductive or non-conductive substrate plated or coated with a suitable conductive material. The inner conductor and outer conductor 104 may be constructed of copper, gold, stainless steel or other conductive metals with similar conductivity values.

Feedline 30 may range in length from about 1 foot (0.3048 m) to about 15 feet (4.572 m), or greater depending on a particular application. In one embodiment, the feedline 30 may be formed from a coaxial, semi-rigid or flexible cable having a wire with a 0.047″ outer diameter rated for 50 Ohms. As depicted in FIG. 1, feedline 30 has a proximal portion 108 operatively connected to, or connectable to, power supply 20 at proximal end 110, and a distal portion 112 that forms a part of microwave antenna assembly 100, as disclosed below. In some embodiments, the feedline 30 and power supply 20 may be a part of an integrated handheld device.

Referring now to FIGS. 1, 3-5, the antenna assembly 100 includes an elongated member 114 disposed about the distal portion 112 of feedline 30, and a sheath 116 that at least partially surrounds a distal portion 102 a (FIG. 3) of the inner conductor 102, as described in further detail below. Elongated member 114 has proximal and distal ends 118, 120 and defines longitudinal axis “A.” Elongated member 114 may be formed of any material suitable for electrically insulating a clinician or operator from the inner and outer conductors 102, 104 of feedline 30 disposed therein such that the antenna assembly 100 may be handled during use.

In non-deployed configuration, the elongated member 114 conceals a distal portion 102 a (FIG. 3) of the inner conductor 102 when the microwave antenna assembly 100 is not in use so as to prevent unintentional damage or injury. In particular, the elongated member 114 conceals the distal portion 112 of feedline 30, which includes distal portions 102 a, 104 a, and 106 a of the inner conductor 102, the outer conductor 104, and the inner insulator 106, respectively. Accordingly, the inner conductor 102, the outer conductor 104, and the inner insulator 106 also constitute components of antenna assembly 100.

At least a portion of the inner conductor 102, i.e. distal portion 102 a, is deployable relative to distal portion 104 a of the outer conductor, such that the antenna assembly 100 may transition from a first, non-deployed configuration (FIG. 3), to a second, deployed configuration during use (FIGS. 4 and 5), as described in further detail below. In the first condition, the distal portion 102 a of the inner conductor is at least partially disposed within the distal portion 104 a of the outer conductor and the elongated member 114. In the second, deployed configuration, the distal portion 102 a of the inner conductor extends at least partially beyond a distal end 120 of elongated member 114, such that contact may be made with the target tissue.

Movement from the first configuration to the second configuration may be facilitated through the use of any suitable mechanism, such as, for example, a deployment assembly 50 (FIG. 1). Reference may be made to commonly-owned U.S. Patent Publication No. 2004/0267156, filed Apr. 4, 2004, for a detailed discussion regarding the components and functionality of deployment assembly 50, the entire contents of which is incorporated herein.

In one embodiment, as seen in FIG. 4, antenna assembly 100 includes a distal portion 102 a of an inner conductor that exhibits a substantially arcuate or curved profile when deployed. FIG. 4 shows the antenna assembly 100 in a partially deployed configuration. Reference may be made to commonly-owned U.S. Pat. No. 7,197,363 for a detailed discussion of the structure of arcuate microwave antenna configurations, the entire contents of which is incorporated herein.

With continued reference to FIGS. 3 and 4, the sheath 116 is disposed about distal portion 102 a of the inner conductor in such a manner so as to define a lumen 128. Sheath 116 may be fixedly, releasably, or slidably connected to distal portion 102 a in any suitable manner including, but not being limited to, welding or adhering, as would be appreciated by one skilled in the art. Sheath 116 has proximal and distal ends 130, 132 defined by the points at which sheath 116 is connected to distal portion 102 a. In one embodiment, as best seen in FIG. 4, the distal-most tip 134 of distal portion 102 a extends beyond the distal end 132 of sheath 116. In another embodiment, the sheath 116 may be connected to the distal portion 102 a of an inner conductor 102 at the distal-most tip 132 thereof, or at a point therebeyond (not shown).

The proximal end 130 of sheath 116 may be located at any suitable location along the length of distal portion 102 a of the inner conductor, dependent upon the desired volume of lumen 128. Although depicted as substantially incisive, the present disclosure contemplates that distal-most tip 134 may be substantially arcuate, duckbilled, or any other such configuration suitable for facilitating the entry of the microwave tissue treatment device into the tissue of a patient.

Sheath 116 may be formed of any suitable biocompatible, impermeable material capable of retaining gas and/or fluid therein, including and not limited to PTFE and tetrafluorethylene-perfluorpropylene (FEP). The present disclosure contemplates that sheath 116 may be either substantially rigid, or substantially non-rigid in character.

Referring back to FIG. 1, the fill source 40 operates in conjunction with, and is fluidly connected to, lumen 128 of sheath 116 such that one or more dielectric materials (e.g., fluids or gases) may be circulated therethrough. The dielectric compounds also serve to dissipate some of the heat generated by the antenna assembly during use in addition to acting as a medium that modifies the dielectric constant of the distal portion of the antenna assembly. Suitable dielectric fluids include, but are not limited to, water, saline, liquid chlorodifluoromethane, or any suitable perfluorocarbon fluid, such as Fluorinert®, distributed commercially by Minnesota Mining and Manufacturing Company (3M™), St. Paul, Minn., USA. Suitable dielectric gases include air, nitrogen, nitrous oxide, carbon dioxide and the like. In yet another variation, a combination of liquids and/or gases may be utilized. The compounds circulated through the lumen 128 may vary depending upon the desired cooling rate and the desired tissue impedance matching properties. The fill source 40 includes a suitable pump configured to supply the dielectric material “M” to the lumen 128. If fluid is being used, the pump may be any type of peristaltic pump and the like. If gas is being used, any type of electric gas pump or compressor may be utilized.

FIG. 5 shows the antenna assembly 100 in a fully deployed configuration, in which the inner conductor 102 fully encompasses a tissue volume “T” targeted for ablation. The inner conductor 102 is formed from a flexible metal suitable to curve about the tissue volume “T” such that the produced ablation volume when the inner conductor 102 is energized by the microwave energy encompasses the tissue volume “T.” The inner conductor 102 may be made from a shape memory alloy, e.g., Nitinol or some other similar alloy, such that as distal portion 102 a is inserted within the tissue, the distal portion 102 a may form the curved and/or helical shape about the tissue volume “T” within the formed ablation volume. The inner conductor 102 extends laterally in relation to the longitudinal axis “A” to define an ablation region that surrounds the tissue volume “T” to be treated in such a manner that the tissue volume “T” is located within the ablation region

In one embodiment, the inner conductor may be formed from a 0.022″ Nitinol wire and the sheath 116 may be formed from PTFE sleeve having an inner diameter of about 0.022″ and an outer diameter of about 0.050.″ The sheath 116 may be inflated using the fill source 40 to adjust the dielectric properties along the length of the inner conductor 102. The sheath 116 may be in a deflated state during the deployment of inner conductor 102 within the tissue. Once inner conductor 102 has been desirably positioned, sheath 106 may be filled with the desired dielectric material “M,” until the sheath 116 has inflated sufficiently about the inner conductor 102. The size of inflated sheath 116 may be varied according to the desired radiative effects, the length of deployed inner conductor 102, as well as the type of tissue.

FIG. 6 shows the cross-sectional view of the antenna assembly 100. The dielectric material “M” is supplied to the lumen 128 to provide a dielectric gap “G” between the inner conductor 102 and the sheath 116. The antenna assembly 100 may be curved, as shown in FIG. 5, to position the gap “G” toward the center of curved antenna assembly 100.

In one embodiment, the lumen 128 may be filled with any dielectric material “M” having a relatively low dielectric permittivity as compared to the material forming the sheath 116. In another embodiment, the lumen 128 may be filled with air, since air has a dielectric constant of 1. A lower dielectric permittivity within the lumen 128 allows for microwave energy to travel through the gap “G” easier than through the sheath 116. Therefore, positioning of the inner conductor 102 in contact with the sheath 116 (e.g., off-center) and curving the antenna assembly 100 such that the gap “G” is facing toward the center of the curved antenna assembly 100 directs the microwave energy toward the center of the looped antenna assembly 100. In particular, this configuration of the gap “G” directs the microwave energy into the looped antenna assembly 100 more efficiently along the inside thereof as opposed to the outside. In other words, this configuration maximizes matching toward the center of the curved inner conductor 102, allowing for better microwave transmission, thereby maximizing ablation within the inner conductor 102. Conversely, this configuration minimizes effects outside the curved inner conductor 102, since the dielectric material “M” of the sheath 116 limits microwave transmission and provides for poor impedance matching between the tissue and the antenna assembly 100.

FIG. 7 shows another embodiment of an antenna assembly 200 having an inner conductor 202 disposed within a multi-lumen sheath 216. The antenna assembly 200 may be curved in a similar manner as the antenna assembly 100 as shown in FIG. 5. The sheath 216 includes two or more lumens 228 and 229 defined therein. The multi-lumen configuration allows for first lumen 228 to provide for a dielectric gap “G” between the inner conductor 202 and the sheath 216 and the inner conductor 202 to be disposed within the second lumen 229. Multi-lumen structure allows for molding of the first lumen 228 to achieve a predetermined shape of the dielectric gap “G” suitable for directing microwave energy into the center of the curved inner conductor 202.

FIG. 8 shows a further embodiment of an antenna assembly 300 having an inner conductor 302 disposed within a sheath 316 having a lumen 328 defined therein. The antenna assembly 300 may be curved in a similar manner as the antenna assembly 100 as shown in FIG. 5. The inner conductor 302 has a substantially hemi-spherical cross-section. This geometry subdivides the lumen 328 to form a dielectric gap “G” between the inner conductor 302 and the sheath 316.

FIG. 9 also shows another embodiment of an antenna assembly 400 having an inner conductor 402 of different geometries. The antenna assembly 400 may be curved in a similar manner as the antenna assembly 100 as shown in FIG. 5. The inner conductor 402 has a substantially U-shaped cross-section. This geometry also subdivides the lumen 428 to form a dielectric gap “G” between the inner conductor 402 and the sheath 416.

The cross-sectional shape of the inner conductors 302 and 402 provide for an enhanced dielectric gap “G.” In particular, the shape of the inner conductors 302 and 402 in combination with the enhanced dielectric gaps “G” provide for directed deposition of microwave energy toward the center of the curved antenna assemblies 300 and 400. In other words, these configurations direct the microwave energy more efficiently along the inside thereof as opposed to the outside.

FIGS. 10-12 show another embodiment of the antenna assembly 500. The antenna assembly 500 includes an elongated member 514 disposed about the distal portion 112 of feedline 30 (FIG. 1), and a sheath 516 that at least partially surrounds a distal portion 502 a of an inner conductor 502. The elongated member 514 is substantially similar to the elongated member 114 and may also be formed of any material suitable for electrically insulating a clinician or operator from the inner conductor 502 disposed therein such that the antenna assembly 500 may be handled during use. The inner conductor 502 may be deployed from within the elongated member 514 similar to the inner conductor 102 as discussed above with respect to FIGS. 3-5.

FIG. 10 shows the antenna assembly 500 in a fully deployed configuration, in which the inner conductor 502 fully encompasses a tissue volume “T” targeted for ablation. The inner conductor 502 is formed from a flexible metal suitable to curve about the tissue volume “T” such that the produced ablation volume when the inner conductor 502 is energized by the microwave energy encompasses the tissue volume “T.” The inner conductor 502 may be made from a shape memory alloy, e.g., Nitinol or some other similar alloy, such that as distal portion 502 a is inserted within the tissue, it may be preconfigured to form the curved shape as the inner conductor 502 is further inserted within the tissue.

The sheath 516 at least partially surrounds the distal portion 502 a of the inner conductor 502 and defines a lumen 528 thereabout. The sheath 516 may be fixedly, releasably, or slidably connected to distal portion 502 a in any suitable manner including, but not being limited to, welding or adhering, as would be appreciated by one skilled in the art. The sheath 516 may be formed of any suitable biocompatible, impermeable material capable of retaining gas and/or fluid therein, including, but not limited to, PTFE and tetrafluorethylene-perfluorpropylene (FEP).

The sheath 516 is formed from a flexible expandable material, such that during inflation, the sheath 516 expands to accommodate the increased volume of the dielectric material “M.” The lumen 528 is in fluid communication with the fill source 40 such that one or more dielectric materials (e.g., fluids or gases) may be circulated therethrough, which are used to inflate the sheath 516.

Suitable dielectric fluids include, but are not limited to, water, saline, liquid chlorodifluoromethane, or any suitable perfluorocarbon fluid, such as Fluorinert®, distributed commercially by Minnesota Mining and Manufacturing Company (3M™), St. Paul, Minn., USA. Suitable dielectric gases include air, nitrogen, nitrous oxide, carbon dioxide and the like. In yet another variation, a combination of liquids and/or gases may be utilized. The selection of dielectric mixtures may be used to provide for better matching microwave energy to different tissue types and sizes.

In addition to the varying dielectric properties of the dielectric compounds, the flexible nature of the sheath 516 also provides for dynamic impedance matching by varying the amount of dielectric material “M” filling the lumen 528. The varying amount of the dielectric material “M” affects the bulk impedance of the antenna assembly 500 based on: (1) the amount of the dielectric material “M” present and (2) the dimension of the sheath 516, which is also based on the amount of the dielectric material “M” present therein.

Based on the dielectric constant of the material, the amount of the dielectric material “M” may be used to increase or decrease the dielectric permittivity of the antenna assembly 500. More specifically, if a dielectric material “M” having a dielectric constant of 2 or more is used then increasing the amount of the material within the lumen 528 reduces the effectiveness of microwave transmission. If a dielectric material “M” having a dielectric constant of 1 or less is used then increasing the amount of such material within the lumen 528 increases the effectiveness of microwave transmission.

The dielectric material “M” may also be used to vary dielectric permittivity of the sheath 516. As shown in FIGS. 11 and 12, the thickness of the sheath 516 varies based on the pressure within the lumen 528 (e.g., amount of the dielectric “M” present therein). Under lower pressure, as illustrated in FIG. 11, the sheath 516 has a relatively large thickness w1. The elasticity of the sheath causes contraction, which increases the wall thickness w1 resulting in a higher overall dielectric constant of the sheath 516. Under higher pressure, as illustrated in FIG. 12, the sheath has a thinner thickness w2. Increasing the pressure within the lumen 528 expands the sheath 516, thereby decreasing the wall thickness w2 while reducing the overall dielectric constant of the sheath 516.

FIGS. 13 and 14 illustrate another embodiment of the antenna assembly 500 having the expandable sheath 516. In particular, the antenna assembly 500 includes a dielectric permeable core 529 within the lumen 528. The core 529 may be formed from any suitable fibrous or porous dielectric material which may be permeable to the dielectric material “M” (e.g., sponge, fiberglass mesh, etc.). The core 529 is secured to the inner conductor 502 and the inner surface of the sheath 516, such that as the sheath 516 is deflated and inflated, the core 529 expands and contracts accordingly, as shown in FIGS. 13 and 14, respectively.

The core 529 provides structural integrity to the antenna assembly 500 by securing the inner conductor 512 at the center of the sheath 516. In addition, the dielectric material of the core 529 provides additional dielectric matching capabilities to the antenna assembly 500. The porous and/or fibrous structure of the core 529 allows the thickness of the sheath 516 to be varied as discussed above with respect to FIGS. 11 and 12. In particular, the thickness of the sheath 516 varies based on the pressure within the lumen 528 (e.g., amount of the dielectric “M” present therein). Under lower pressure, as illustrated in FIG. 13, the sheath 516 has a relatively large thickness w1. The elasticity of the sheath causes contraction, which increases the wall thickness w1 resulting in a higher overall dielectric constant of the sheath 516. In the deflated configuration, the core 529 in combination with the thickened sheath 516 act as the primary dielectric buffers. Thus, the core 529 may be formed from a dielectric material that is suitable for impedance matching the antenna assembly 500 in the deflated state.

Under higher pressure, as illustrated in FIG. 14, the sheath has a thinner thickness w2. Increasing the pressure within the lumen 528 expands the sheath 516, thereby decreasing the wall thickness w2 while increasing the volume of the lumen 528. This, in turn, reduces the overall dielectric constant of the sheath 516.

FIG. 15 illustrates a flow chart of a method for varying the dielectric properties of the antenna assembly 500. In step 600, the antenna assembly 500 is inserted into tissue and is deployed to surround the tissue volume “T” as shown in FIG. 10. In step 602, the sheath 516 is inflated to a predetermined volume such the sheath 516 is stretched to the thickness w2. The thinner thickness w2 provides for a lower dielectric permittivity, thereby providing for optimum impedance matching with undesiccated tissue. In step 604, the antenna assembly 500 is energized to ablate the tissue volume “T.” As a result of the energy application, the tissue volume “T” is desiccated and the impedance thereof increases accordingly. In step 606, the sheath 516 is deflated by withdrawing the dielectric material “M.” As the sheath 516 is deflated, the pressure is decreased, increasing the thickness w1 of the sheath 516, thereby increasing the dielectric constant of the sheath 516 to provide for better dielectric matching with desiccated tissue. During step 606, microwave energy may be continuously supplied to the antenna assembly 500. In step 608, additional microwave energy is supplied to the antenna assembly 500. Steps 606 and 608 may be repeated multiple times to provide for step-down adjustments of the dielectric permittivity of the antenna assembly 500.

The described embodiments of the present disclosure are intended to be illustrative rather than restrictive, and are not intended to represent every embodiment of the present disclosure. Various modifications and variations can be made without departing from the spirit or scope of the disclosure as set forth in the following claims both literally and in equivalents recognized in law. 

1-19. (canceled)
 20. A microwave antenna assembly comprising: a feedline including an inner conductor, an outer conductor, and an insulator disposed between the inner and outer conductors, the inner conductor configured to at least partially encircle target tissue; and an expandable sheath defining a lumen configured to receive a dielectric material to expand the expandable sheath, at least a portion of the inner conductor disposed off-center within the lumen, the inner conductor in contact with a first portion of the expandable sheath and defining a dielectric gap between the inner conductor and a second portion of the expandable sheath, the dielectric gap configured to face the target tissue when the inner conductor at least partially encircles target tissue.
 21. The microwave antenna assembly according to claim 20, wherein the expandable sheath is disposed about a distal portion of the inner conductor.
 22. The microwave antenna assembly according to claim 20, wherein a distal end of the expandable sheath is disposed proximal to a distal end of the inner conductor.
 23. The microwave antenna assembly according to claim 20, wherein the expandable sheath is formed of a material selected from the group consisting of polytetrafluoroethylene and tetrafluorethylene-perfluorpropylene.
 24. A microwave ablation system, comprising: a microwave antenna assembly including: a feedline including an inner conductor, an outer conductor, and an insulator disposed between the inner and outer conductors; an elongated member disposed about the feedline, the inner conductor configured to deploy from a distal end portion of the elongated member to at least partially encircle target tissue; and an expandable sheath defining a lumen, the inner conductor disposed off-center within the lumen to define a dielectric gap between the inner conductor and the expandable sheath, the dielectric gap configured to face the target tissue when the inner conductor is deployed from the distal end portion of the elongated member; and a fill source coupled to the lumen and configured to deliver dielectric material to the lumen.
 25. The microwave ablation system according to claim 24, wherein the fill source includes a pump configured to circulate the dielectric material through the lumen.
 26. The microwave ablation system according to claim 24, where at least a portion of the inner conductor is in contact with the expandable sheath.
 27. The microwave ablation system according to claim 24, wherein the expandable sheath is disposed about a distal portion of the inner conductor.
 28. The microwave ablation system according to claim 24, wherein a distal end of the expandable sheath is disposed proximal to a distal end of the inner conductor.
 29. The microwave ablation system according to claim 24, wherein the expandable sheath is formed of a material selected from the group consisting of polytetrafluoroethylene and tetrafluorethylene-perfluorpropylene.
 30. A method for performing microwave ablation, the method comprising: inserting a microwave antenna assembly into tissue, the microwave antenna assembly including an expandable sheath defining a lumen and an inner conductor disposed off-center within the lumen; at least partially encircling target tissue with the inner conductor; delivering a dielectric material to the lumen to expand the expandable sheath; and energizing the microwave antenna assembly to ablate the at least partially encircled target tissue.
 31. A method according to claim 30, further comprising withdrawing the dielectric material from the lumen to contract the expandable sheath.
 32. The method according to claim 30, wherein delivering the dielectric material to the lumen creates a dielectric gap between the inner conductor and the expandable sheath. 